The failed action had threatened peace negotiations between the government and the rebels

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BAMAKO, May 29 (Reuters) - Mali's army chief of staff, General Mahamane Toure, offered to step down on Thursday following the military's failed attempt to seize the Tuareg separatist town of Kidal, three military sources said.

General Toure, if his resignation is accepted by President Ibrahim Boubacar Keita, will be the second senior official to resign after Defence Minister Soumeylou Boubeye Maiga quit on Tuesday following fighting in which 50 Malians were killed.

"He tendered his resignation letter to the president this morning. We do not have an answer yet, we do not know if it will be accepted or not," one military source close to the general told Reuters.

Another military source close to the Malian presidential office confirmed the army chief had offered to resign.

The Malian army launched an assault on Kidal last Wednesday after clashes broke out in the northern town during a visit by Prime Minister Moussa Mara. The government has said President Keita did not give the order for the attack.

Mali's army was quickly overrun by rebel forces, who said they subsequently captured seven other northern towns as U.N. and French peacekeepers declined to intervene.

The failed action had threatened to sink struggling peace negotiations between the government and the rebels, and plunge the impoverished West African country back into war. A ceasefire was later brokered by the African Union and the United Nations.